Where they often occur ? Why ?

Most of the earthquakes occurred at subduction zones (where an oceanic plate is being subducted underneath either another oceanic plate or a continental plate). This is not the type of environment under which the Lewis thrust was active. The earthquakes in the Andes, the Himalayas, northern Africa, and throughout Iran occurred along faults that are similar to the Lewis thrust. There are also many active thrust faults in southern California, and the Coast and Transverse Ranges were uplifted along such faults. This figure indicates that not only are thrust faults physically possible, they are very common.